Combustion of high-sulfur coal with minimal ecological trauma

ABSTRACT

Heating values are recovered from sulfur-containing coal in a two-stage process which minimizes pollution. In the first stage, sulfur-containing coal is reacted with CaO in a fluidized bed gasifier which operates adiabatically, under reducing conditions, to yield CaS and a gas rich in CO. The second stage is a conventional boiler to which the off-gas from the first stage and entrained desulfurized coal fines, are delivered as fuel. The flue gas from this boiler contains substantially no sulfur dioxide as a pollutant. CaS is withdrawn from the first stage as a valuable byproduct.

United States Patent Philadelphia, Pa.

MINIMAL ECOLOGICAL TRAUMA 72] Inventor Marshall L. Spector Princeton, NJ. [2| Appl. No. 60.379 [22l Filed Aug. 3, i970 I45] Patented Aug. l7, l97l I73] Assignee Air Products and Chemicals, Inc.

- [54] COMBUSTION OF HIGH-SULFUR COAL WITH [56] l j Ram-mes Cited Primary Examiner-Kenneth W. Sprague Attorneys-Barry Moyerman and B. Max Klevit ABSTRACT: Heating values are recovered from sulfur-containing coal in a two-stage process which minimizes pollution.

in the first stage, sulfur-containing coal is reacted with CaO in a fluidized bed gasifier which operates adiabatically, under reducing conditions, to yield CaS and a gas rich in CO. The second stage is a conventional boiler to which the off-gas from the first stage and entrained desulfurized coal fines, are delivered as fuel. The flue gas from this boiler contains substantially no sulfur dioxide as a pollutant. CaS is withdrawn from the first stage as a valuable byproduct.

10 Chile, 3 Drawing Figs.

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, 110/3! [51] Int,Cl. Cl0j 1/00 [50] Fieldolselrch... 122/4, 5; 110/7, 8,3!

l I l7 0 0 0 on s? PATENTEU AUG] 71971 3.599.610

sum 1 OF 2 -Fig.|

K INVENTO Marshall L.Spec1or ATTORNEY MOLE RATIO OF CO/CO PATENTEUAUBI Hen 3599.610

SHEET 2 BF 2 Fig. 3

APPROXIMATE MINIMUM MOLE RATIO OF CO/CO NECESSARY IN OFF'GAS TO PREVENT OXIDATION OF 008 TO C0804 GASIFIER TEMP. F

INVENTOR Marshall L. Specfor COMBUSTION or mon-suuua COAL wrrn MINIMAL ECOLOGICAL TRAUMA removed-as calcium sulfide.

, 2. Prior Art The prior art is legion and it would indeedbe presumptuous to attempt a summary herein which would be both comprehensive and objective.

Combustion of coal in fluidized beds is not new. It is shown, for example, in U.S. Pat. No. 3,437,561. Recent work along these lines, done by the British Coal Utilization Research Association, is described in an article by S. Wright et al. "T11. of the Institute of 'Fuel 42, 235-439, June I969. There have also been prior attempts to recapture the sulfur content of coal, as calcium sulfate, in thecourse of single-stage combustion of coal in fluidized bed reactors. These were described, variously, by E. B. Robison et a]. in Fluidized No. 4 (Apr. I948) pg. 562 et seq.

The recent emphasis on preservation of our environment has now made it extremely desirable to obtain heat values from'carbonaceous fuels without polluting the atmosphere with sulfur dioxide. While one obvious solution is to utilize fuels which contain little or no sulfur, such fuels are scarce and relatively expensive. Prior art technology, even that devoted ,to re'moval of sulfur from carbonaceous fuels, was more. concerned with upgrading the quality of the carbonaceous material than with minimizing the ecological trauma associated with its combustiomThe treatment or pretreatment of such materials with various sulfur acceptors'f or getters" is discussed inter alia, in U.S.Pat. Nos. 2,824,047 and 3,387,941. However, inthc former. regeneration of .the acceptor yields SO, and, in the latter, sulfur-derived impurities are I released in gaseous form. The'general technology of attempts to desulfurize fuels undergoing gasification is reviewed in an article by, A. M. Squires in Preprints. Div. of Fuel Chemistry A.C. S. Vol. l No. 4 pg. -4I (I966), which discloses, inter alia, the-use of .dolomite for desulfurization. Said article is incorporated herein by reference.

Sufficeit to say that the prior art, while certainly possessed of all the manipulativc techniques and'equipment technology 2' I limestone (used herein to mean any Ca0-yielding material) and coal. Adiabatic conditions are maintained by balancing theexotherm of the oxidation of carbon to CO and the endotherm of calcination of CaCO and the heat duty required to heat raw'materials to reaction temperatures. Additional control of temperature may be obtained by preheating the 'fluidizing gas or by the injection into it of water or carbon dioxide. Reactor temperatures range from 1,650 F. to about Products of the gasifier are both gaseous and solid. CaS is drawn off from the bed as a product from which, if desired. elemental sulfur can be readily recovered. This CaS is relatively free of coal ash since'the bulk of the ash is elutriatedfrom the gasifier. The off-gas is rich in C0. Entrained in this gas are fly ash and substantially sulfur-free coal fines from the reactor, which have reached a size where they.,ca n.no longer be retained in the bed. The second stage is a conventional boiler. The off-gas and entrained solids such as, forexample, coal from the reactor are fed directly to it as fuel, preferably isothennally, along with secondary combustionv air. The economics of the process are enhanced by maximizing the amount of desulfurized coal solids carried over to the boiler. Flue gas from this boiler consists primarily of CO H 0 and N Usual waste heat recovery techniques may be employed for increased thermal efiiciency.

Accordingly, it is an object of the invention to provide a I method of recovering heat values from sulfur-containing coal needed for practice of the instant invention has never produced a'two-stage process for combustion of sulfur-containing coal wherein the first stage is a fluidized-bed adiabatic gasifier in which calls reacted with CaO, which'may be derived fro m'limestone or dolomite, under reducingconditions, to produce-mot'CaSO -but rather CaS and wherein the off-gas plus entrained solids are fed as fuel to a conventional boilerwhose flue gas is essentially sulfur-free.

SUMMARY OF THE INVENTION which minimizes emission of pollutants, which provides CaS as a byproduct, which is efficient in operation and economically viable. I I

Other objects of the invention will be apparent to those skilled in the art from a consideration of the description which follows, when read in conjunction with the accompanying drawings. i A

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein like reference numerals designate like parts: A I

-.FIG. 1 is a schematic flow diagram of one embodiment of the invention.

FIG. 2 is a schematic flow diagram of another embodiment of the invention, differing from that shown in FIG. 1, by virtue Y of the use of added water in the gasifier.

FIG. 3 is a graph illustrating the minimum CO/CO ratio in the off-gas which must be maintained, at given gasifier temperatures, to produce CaS rather than CaSO.,.

, DESCRIPTION OF THE FREF ERRED EMBODIMENTS bustion chamber 21 and a stack v22, both of conventional tions. 'By' oper'ating under reducing conditions, formation of I CaSO, and oxides of sulfur is prevented.

The reactor contains a steady-state mixture of presized design. The overhead effluent from reactor 10 is fed to combustion chamber 21 via transfer line 17, Secondary combustion air is supplied, for example, via line 18 which feeds into the boiler.

Typical operation of the embodiment shown in FIG. 1, is based on I ton (i.e. 2,000 pounds) of gasified coal having a sulfur content of 4.5 weight percent with the gasifier operating at a temperature of about 1,840 F. This basis does not include additional coal fines elutriated'from the bed. The feed to the bed 12, which entersvia line 16, comprises a mixture of coal and limestone totaling about 2,420 pounds. Material enters at ambient temperature (e.g. 77 F.) and its analysis in pounds and pound-moles is asfollows:

temperatures, on the order of l650 F. Typical of such an operation, again on the basis of a ton of gasified coal with a sulfur content of 4.5 percent and 9 moles of water fed through line 24 is the following. Feed through line 1.6:

e W W -Mm. Q

O-Moles 0 O-Molcs cell: 0 L600 133 c [,600 I33 3505 8 6953 323.8 H,S :25! 32.8 Ash 242 2.0 (as 2 2 242 2D Limestonet CICO', 420 Caco: 560 576 2,420 17s Air, used as the fluidizing gas, enters through line 14, to pro Air is supplied through line 14, equal to 61 pound-moles of 0x vide 64 pound-moles of O, and 221 pound-moles of N ygen and 230 pound-moles of nitrogen. The material Thebed 12 itself contains a steady-state mixture of about 90 withdrawn through line 15 comprises 2.5 pound-moles of CaS percent limestone and 10 percent coal. The sizeof the 15 and 0.4 pound-moles ofCaSiO in addition to some unreacted limestone is adjusted to avoid its excessive elutriation from the lime and coal. 5 bed. Lime carryover is minimal (i.e. below 2 percent of inven- In this example the overhead gas, moving through line 17, C tory) ifparticulate lime orlime-yielding material is charged om rises m having a size range from 2 to 3 mm; in diametcrand the supert'lcial linear gasvelocity through the bed is held below l4 M40," fL/sec'ond, preferably about 5 ft./second.

The coal ispreg'round so that 99 percent has a size less than Co I 34 4 mesh. As gasification proceeds, a given particle will be con- 43 sumed until, ultimately, it leaves through line 17 elutriated in 1 1: H the off-gases. The maximum size of coal particles which will be elutriated is a function of the linear velocity in the bed. Op- 230 timum conditions are achieved when the coal has a residence W time in the bed just long enough to substantially eliminate its Flue gas leaving stack 22 under these operating conditions will sulfur content'and, after this time has elapsed is at elutriable have the following cqmposition:, M v particle size. Carryover of-5'20 percent of coal tines from WWW" M m V I the gasifier is usual. Greater carryover will, of course,- reduce 6 operating costs-However, in this milieu, carryover is limited by the desiderata that: (a) the gasifier temperature be main- Co, I36 tained within the range where the desired desulfurization-reac 44 tion occurs and (b) thesulfur content of the fines is within the 1 563 generally recognized range for low-sulfur carbonaceous fuels (e.g. less than l-percent by. weight). If these guidelines are fol- T;d'- :y%"':" ig ig ii l lm l a f ii tf f The keyto the success of the process is the operation of the s: 2 :22:; g f sf fi s i g g g s 40 gasifier under reducing conditions. Such conditions are a function of the ratio of coal to oxygen after steady-state operad Pound-moles of ash expressed as m addmon tion has been achieved. This ratio is, in turn,.reflected in the "F'Zgf'gjfifi g g if? f co/co ratio in the gasifier effluent R'eferrin g to FIG. 3, P'Z' 7 m m c co may 50 e operation should always be above the line to mamtam reducn g g g' g g g s zz f z if?" ing conditions in the gasifier. Accordingly, feed of air and coal :s pgsi 5 2 i iz f z g e e or must be ad usted sothat the CO/CO; ratio |s represented by a f The overhead leaving through line 17 contains entrained qt *apqv'e'flge.lmggnnogi. coal fines, as stated above, and some CaS despite the presence The limestone mulled always m F of that necessary of diengagem'em space 3' The gasedus components to supply CaO to react wlth the sulfur in the coahMolar ex- PH 136 pouncbmoles of C0, 35 poundqnoks of H2, 02 cesses of 50 to 200 percent are preferable, with an optimum pound-moles of H,O, 0.4 pound-moles of C0,, 221 pound- -range 75 to i moles of Ngand traces of H,S. This producer gas (i.e. gas rich whfle two i' l of the have been in Co and H) fl at substantially constant temperature to described, many modifications and variations w ll be apparent boiler 21 in line 17. Secondary air, which is preferably pre- 9 those f respect 9 the design and operaheated in conventional equipment enters through line 18. The P f flufdlzed'be'd g ingly, the scope of the air supplies 322 pound moles of N: and 86 pound moles ofoz invention 18 to be limited only by a reasonable interpretation Flue gas coming out of stack. 22 contains 136 pound-moles of the ,lppended claims C0,, 35 pound-moles of H,O, 563 pound-moles of N, and 0.5 damn v pound-moles of S0,. The process has thus performed as if fuel A prosiess F values t f" having 0.8 percent sulfur were conventionally burned whereas coal wh'le the yield of Sulfur dloxlde compns' the fuel utilized contained, in fact, 4.5 percent sulfur. mg the stein 9 v The embodiment of the invention shown in FIG. 2 is establishmg a bed or pamgula-tqcoal and limestone; generally similar to the previous embodiment. It includes, fluidiling Said bed with a Stream of air containing enough however, a .water vaporizer 23, operatively associated with yg to Pmduce CO from the C03]; boiler 21. Water flows to this heat exchanger (i.e. vaporizer) feeding Said below the P thereof, with fluidilflblethrough line 24 and exits, as'superheated steam through line S Sulfur-Containing C081;

25. The steam is added to reactor 10 to help control the exd. feeding saidbed with anamount of limestone in excess of I otherm. CO, can be used in lieu of or in addition to water for' that needed to produce CaS from the sulfur contained in this purpose. I the coal;

Operating in this manner, the reactor can be run at lower e. reacting carbon with oxygen in said bed, under adiabatic conditions, in a temperature range of from about 1650 F.

to about 2100 F.; adjusting the feed of air and coal to prevent oxidation of CaS to CaSO, by producing a reducing environment in the bed, as reflected by the CO/CO, ratio in the gaseous effluent therefrom, said ratio having a value above the minimum represented by the curve in FIG. 3;

g. drawing off particulate CaS from the bed;

' h. removing from the bed, as an overhead effluent, producer gas in which is entrained a quantity of fine coal, said fine coal having a sulfur content substantially less than that of the coal originally fed;

i. transferring said producer gas and its entrained solids to the inlet of a boiler;

j. burning said producer gas and said entrained coal, with the addition of secondary air, in said boiler to recover the heat value thereof;

whereby the'stack gas emitted from said boiler will contain less SO, than it would, had the original coal been fed directly thereto.

2. The process of claim 1 wherein the amount of limestone fed contains from 50 to 200 mol percent excess of Ca.

3. The process of. claim 1 wherein 99 percent of the coal fed to the bed has a size less than 4 mesh.

4. The process of claim 1 wherein the producer gas is transferred to the boiler isothermally.

5. The process of claim 1 where a fluid selected from the group consisting of water and CO is added to the fluidized bed to control the exotherm thereof.

6. The process of claim 2 wherein 99 percent of the coal fed to the bed has a size less than 4 mesh.

7. The process of claim 2 wherein the producer gas is transferred to the boiler isothermally.

8. The process of claim 2 wherein a fluid selected from the group consisting of water and CO is added to the fluidized bed to control the exotherm thereof.

9..The process of claim 3 wherein a fluid selected from the group consisting of water and CO is added to the fluidized bed to control the exotherm thereof.

10. The process of claim 9 wherein the producer gas is transferred to the boiler isothermally.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Fatent No. 3,599,610 Dated August 17, 1971 Inventor) Marshall L. Spector 1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 17, after "bed" Start a new paragraph. 1

Column 3, line 1, delete "'o" and substitute --Pound-- Column 3, line I, delete "O-Moles and substitute --Pound-Moles--,

Column 3, line 4, insert parenthesis after "(as H 8" Column 3, line 6, after "0 insert parenthesis Column line 1, delete "O" and substitute -Pounds-- Column line 1, delete O-Moles and substitute --Pound-Moles-- Column 4, line 20, delete "O-Moles" and substitute --Pound-Moles-- Column '4, line 31, delete "O-Moles" and substitute --Pound-Mo1es-- Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETC HER JR. ROBERT GOTTSCHALK LAttSStlflg Officer Commissioner of Patents J 

1. A process for recovering heat values from sulfur-containing coal while minimizing the yield of sulfur dioxide, comprising the steps of: a. establishing A bed or particulate coal and limestone; b. fluidizing said bed with a stream of air containing enough oxygen to produce CO from the coal; c. feeding said bed, below the top thereof, with fluidizablesize sulfur-containing coal; d. feeding said bed with an amount of limestone in excess of that needed to produce CaS from the sulfur contained in the coal; e. reacting carbon with oxygen in said bed, under adiabatic conditions, in a temperature range of from about 1650* F. to about 2100* F.; f. adjusting the feed of air and coal to prevent oxidation of CaS to CaSO4 by producing a reducing environment in the bed, as reflected by the CO/CO2 ratio in the gaseous effluent therefrom, said ratio having a value above the minimum represented by the curve in FIG. 3; g. drawing off particulate CaS from the bed; h. removing from the bed, as an overhead effluent, producer gas in which is entrained a quantity of fine coal, said fine coal having a sulfur content substantially less than that of the coal originally fed; i. transferring said producer gas and its entrained solids to the inlet of a boiler; j. burning said producer gas and said entrained coal, with the addition of secondary air, in said boiler to recover the heat value thereof; whereby the stack gas emitted from said boiler will contain less SO2 than it would, had the original coal been fed directly thereto.
 2. The process of claim 1 wherein the amount of limestone fed contains from 50 to 200 mol percent excess of Ca.
 3. The process of claim 1 wherein 99 percent of the coal fed to the bed has a size less than 4 mesh.
 4. The process of claim 1 wherein the producer gas is transferred to the boiler isothermally.
 5. The process of claim 1 where a fluid selected from the group consisting of water and CO2 is added to the fluidized bed to control the exotherm thereof.
 6. The process of claim 2 wherein 99 percent of the coal fed to the bed has a size less than 4 mesh.
 7. The process of claim 2 wherein the producer gas is transferred to the boiler isothermally.
 8. The process of claim 2 wherein a fluid selected from the group consisting of water and CO2 is added to the fluidized bed to control the exotherm thereof.
 9. The process of claim 3 wherein a fluid selected from the group consisting of water and CO2 is added to the fluidized bed to control the exotherm thereof.
 10. The process of claim 9 wherein the producer gas is transferred to the boiler isothermally. 